1. Field of the Invention
This invention relates to abrasive products comprising abrasive particles, binder, and an inorganic metal orthophosphate salt, and to methods of making and using same. These abrasive products include bonded abrasives, coated abrasives, and nonwoven abrasives.
2. Description of the Related Art
In the competitive and economically significant field of abrasive products, a continuing desire exists to reduce manufacturing costs and increase performance of such products in efforts to seek and acquire competitive edge.
Abrasive products are generally known having abrasive particles adherently bonded to a sheet-like backing. It is generally known to stratify the abrasive grains and binders into separate layers that are serially formed upon a sheet-form substrate, such as in coated abrasive articles, in such a way as to basically segregate the abrasive grains as a particulate monolayer sandwiched between underlying and overlaying binder layers.
More specifically, coated abrasive products typically have a backing substrate, abrasive grains, and a bonding system which operates to hold the abrasive grains to the backing. In a typical coated abrasive product, the backing is first coated with a layer of adhesive, commonly referred to as a "make coat", and then the abrasive grains are applied to the adhesive coating. The application of the abrasive grains to the make coat involves electrostatic deposition or a mechanical process which maximizes the probability that the individual abrasive particles are positioned with its major axis oriented perpendicular to the backing surface. As so applied, the abrasive particles optimally are at least partially embedded in the make coat. The resulting adhesive/abrasive grain layer is then generally solidified or set (such as by a series of drying or curing ovens) sufficient to retain the adhesion of abrasive grains to the backing. After curing or setting the make coat, a second layer of adhesive, commonly referred to as a "size coat", is applied over the surface of the make coat and abrasive particles, and, upon setting, it further supports the particles and enhances the anchorage of the particles to the backing. Optionally, a "supersize" coat, which may contain grinding aids, can be applied over the cured size coat. In any event, once the size coat and supersize coat, if used, has been cured, the resulting coated abrasive product can be converted into a variety of convenient forms such as sheets, rolls, belts, and discs. As an optional supersize enhancement, to mitigate any anticipated loading or clogging of the abrasive product with swarf (i.e., debris liberated from the workpiece during the abrading operation), a coating of anti-stick stcaratc also can be applied as supersize over the exterior of the abrasive coating, once formed, as suggested in Kirk-Othmer Encyclopedia of Chemical Technology, Fourth Ed., Vol. 1, (p. 29).
In many abrasive articles the binder includes a particulate filler as an adjuvant. Typically, the binder will comprise between 40 to 70 percent by weight particulate filler. The addition of the filler either increases the toughness and hardness of the binder and/or reduces the cost of the finished article, e.g., by decreasing the amount of binder required. The filler is typically an inorganic particulate material, generally having a particle size less than about 40 micrometers. Examples of common fillers in the abrasive industry include calcium carbonate, calcium oxide, calcium metasilicate, alumina trihydrate, silica, kaolin, quartz, and glass.
There exists a subclass of fillers, referred to as grinding aids, cutting aids, or generically as "active fillers". An active filler is typically a particulate material the addition of which to the binder has a significant affect on the chemical and physical processes of abrading which leads to improved performance. It is believed that active fillers will either (1) decrease the friction between the abrasive grains and the workpiece being abraded, and/or (2) prevent the abrasive grains from "capping", i.e. prevent metal particles from becoming welded to the tops of the abrasive grains, and/or (3) decrease the interface temperature between the abrasive grains and the workpiece, and/or (4) decrease the required grinding force.
Grinding aids can be especially effective in abrading stainless steel, exotic metal alloys, titanium, metals slow to oxidize, and so forth. In some instances, a coated abrasive product containing a grinding aid in the binder can abrade up to 100% more stainless steel than a corresponding coated abrasive product in which the binder is devoid of a grinding aid. The reason, in theory, being that the activity of grinding metal by abrasive articles produces freshly formed, hot, and uncontaminated metal surfaces. If the newly formed, uncontaminated metal surface is not rapidly "contaminated", metal will transfer and adhere to the abrasive particle surface(s) causing "capping" which decreases grinding performance. One purpose and function of grinding aids is to prevent capping by rapidly contaminating the freshly formed metal surface. Grinding aids are normally incorporated into the bond resin(s) of the abrasive article. Grinding aids (active fillers) can be classified as physically active or chemically active. Cryolite, sodium chloride, and potassium tetrafluoroborate are known physically active grinding aids that melt between 500 and 1,000.degree. C. which can form thin films on freshly formed metal. Chemically active grinding aids include iron pyrite, polyvinyl chloride, and polyvinylidene chloride which decompose when heated forming chemicals that rapidly react with the freshly formed metal surface.
Also, combinations of grinding aids in abrasive articles (grinding wheels) may produce more than a cumulative grinding effect. U.S. patents describing use of the combination of a sulfide salt and an alkali metal salt include U.S. Pat. Nos. 2,408,319; 2,811,430; 2,939,777; 3,246,970; and 5,061,295. Other patents that combine an inorganic salt containing fluorine, c.g. cryolite, and a salt such as ammonium chloride include U.S. Pat. Nos. 2,949,351 and 2,952,529.
Another type of grinding aid enhancement is described in U.S. Pat. No. 5,441,549 (Helmin) wherein the grinding aid effect of potassium tetrafluoroborate is enhanced by the addition of specific thermoplastics.
Other descriptions of grinding aids include:
U.S. Pat. No. 2,216,135 (Rainier), which teaches a grinding wheel having as a grinding aid an anhydrous, water-soluble non oxidizing inorganic alkali or alkaline earth metal salts whose melting points are within the range of 700 to 1200.degree. C. These materials include sodium chloride, potassium chloride, anhydrous sodium carbonate, sodium sulfate, potassium sulfate, lithium sulfate, sodium pyrophosphate, potassium pyrophosphate, calcium chloride, calcium bromide, magnesium sulfate, barium chloride, barium bromide, magnesium chloride, magnesium bromide or strontium chloride.
U.S. Pat. No. 2,243,049 (Kistler), which teaches an abrasive body (grinding wheels) containing finely divided strongly acidic or potentially acidic inorganic compounds. Acid sulfates, phosphates or pyrophosphates are satisfactory, as are the ammonium, sodium, potassium, calcium, or barium salts thereof. Phosphorus pentoxide is also possible. The grinding aid constitutes about 7% of the bond. is When used on metal work surfaces, the grinding aid reduces loading and increases the grain efficiency 40 to 100%.
U.S. Pat. No. 3,502,453 (Baratto) discloses abrasive articles containing hollow spheriles filled with lubricant, which spherules rupture during grinding to release the lubricant. In one example, Baratto discloses a formulation molded into a wheel for titanium snagging, where the formulation includes silicon carbide, bonding resin, trisodium phosphate, and encapsulated lubricant.
U.S. Pat. No. 2,690,385 (Richlin), which teaches a metal cleaning cloth or felt impregnated with abrasive, sodium bisulfate and a humectant. Substitutes for the sodium bisulfate include ammonium chloride, ammonium phosphate, aluminum chloride, antimonious chloride, potassium bisulfate, oxalic acid, phosphoric acid and tartaric acid.
U.S. Pat. No. 3,030,198 (Kibbe), which discloses a grinding wheel containing potassium hexafluorophosphate as a grinding aid.
U.S. Pat. No. 3,032,404 (Douglass et al.), which discloses a grinding wheel containing as a grinding aid finely divided solid heavy metal phosphide. It is preferable to also include potassium aluminum fluoride in the grinding wheel.
U.S. Pat. No. 3,770,401(Sheets et al.), which describes an abrasive body (grinding wheel) comprised of grit-sized particles of alumina or silicon carbide held together by a water-insoluble aluminum phosphate bonding matrix.
U.S. Pat. No. 5,096,983 (Gerber), which teaches the use of up to 5.0% of a water soluble salt such as sodium phosphate to retard the room temperature and eventual hardening of phenolic resole resins which are mixed with magnesium oxide with or without an ester functional hardening agent.
U.S. Pat. No. 5,116,392 (Selgrad et al.), which teaches a grinding aid having the formula: uM.sub.1.multidot.M.sub.2.multidot.wHal.multidot.xChal.multidot.zPh, where M.sub.1 is a pure metal or mixture of alkali metal, alkaline earth metal and/or Al; M.sub.2 is a pure metal or mixture of Zn, Mn, Fe except for Fe as chloride; Hal is a pure halogen or mixture of F, Cl, Br, I; Chal is chalcogenidcs, O and/or S; Ph is phosphate or more highly condensed phosphates of the formula P.sub.r O.sub.s where r=1 to 10, preferably 1 to 2, s=4 to 20, preferably 4 to 7; and u, v, w, x or z=0 to 95%.
PCT application WO 95/24992 (Ho et al.), describes abrasive articles having a peripheral (outermost) coating comprised of grinding aid particles and a binder, where the grinding aid particles are individually coated with an inert, hydrophobic, hydrocarbon-containing substance. For coated abrasive articles, the peripheral coating is stated to refer to either the size or supersize coat that is the outermost coating on the abrasive surface of the article. The individually-coated grinding aid particles also may be incorporated into erodible grinding aid agglomerates, with a binder to adhere the grinding aid particles together, and these agglomerates can be incorporated into the make, size and/or supersize coats of a coated abrasive. Although a number of examples of grinding aid particles are disclosed in PCT application WO 95/24992, alkali or alkaline earth metal phosphates are not named.
PCT application WO 97/14534 (Harmer et al.), filed on even date with the present application, describes abrasive articles having an alkali or alkaline earth metal metaphosphate, such as sodium metaphosphate, in the peripheral coating layer, and methods of making these abrasive articles, as well as a method of using them to grind titanium.
U.S. Pat. No. 5,702,811 (Ho et al.) describes coated abrasive articles having an abrasive grain layer formed in a make coat, which, in turn, is coated with a size coat or a size coat and a super size coat, where the abrasive grain layer is comprised of abrasive grains and nonabrasive composite grains which contain inorganic nonabrasive particles bonded together by a metal salt of a fatty acid or colloidal silica, or combinations thereof.
Titanium alloys, in particular, such as those designed for aerospace applications and other applications where high strength to weight ratios are desirable, are extremely difficult to grind, even with conventional grinding aids. Although the high strength of these alloys is a major cause of poor grindability, chemical adhesion of the titanium to the abrasive grain is also thought a factor contributing to poor abrasive performance. These difficulties can be alleviated somewhat by use of certain grinding fluids, such as coolants or lubricants, used to flood the grinding interface between the abrasive article and workpiece. Materials used as grinding fluids for titanium include soluble cutting oils such as highly chlorinated cutting oils and buffered inorganic tripotassium phosphate solutions, the latter of which being described by I. S. Hong et al., "Coated abrasive machining of titanium alloys with inorganic phosphate solutions", Trans. ASLE, 14 (1971), pages 8-11. Additionally, a comparative study of grinding aid lubricants involving the use of among four inorganic salts NaNO.sub.2, KNO.sub.2, Na.sub.3 PO.sub.4, and K.sub.3 PO.sub.4, is described by Caldwell et al., "Grinding a titanium alloy with coated abrasives," ASME Paper 58-SA-44, June, 1958. Although widely used in buffered solutions, the tripotassium phosphate salts have proven difficult to incorporate into resin-bonded systems due to their hygroscopic nature.
U.S. Pat. No. 4,770,671 (Monroe et al.) describes adding various types of grinding aids onto the surface of alpha-alumina-based ceramic abrasive grits in coated abrasives. In one example, Monroe et al. describe K.sub.2 HPO.sub.4 as a grinding aid.
A variety of "phosphates" exist as salts of acids of phosphorus. The conventional nomenclature and associated chemical formulae of several common anions for these salts include the following:
orthophosphate=PO.sub.4.sup.-3 PA1 monohydrogen orthophosphate=HPO.sub.4.sup.-2 PA1 dihydrogen orthophosphate=H.sub.2 PO.sub.4.sup.-1 PA1 metaphosphate=PO.sub.3.sup.-1 PA1 pyrophosphate=P.sub.2 O.sub.7.sup.-4. PA1 (a) applying a first binder resin precursor to a substrate; PA1 (b) at least partially embedding a plurality of abrasive particles in said first binder resin precursor; PA1 (c) at least partially curing said first binder resin precursor; PA1 (d) applying a second binder resin precursor over said at least partially cured first binder resin precursor and said plurality of abrasive particles; PA1 (e) at least partially curing said second binder precursor resin precursor; PA1 (f) applying a third binder resin precursor and an inorganic metal phosphate salt devoid of hydrogen selected from the group consisting of an alkali metal orthophosphate and an alkaline earth metal orthophosphate; and PA1 (g) completely curing said first, second and third binder precursor resin precursors.
This terminology is applicable for purposes of this application.